Seventeen years ago, astronomers spotted the brightest stellar explosion ever seen since the one observed by Johannes Kepler 400 years ago. Called SN 1987A, the titanic supernova explosion blazed with the power of 100,000,000 suns for several months following its discovery on Feb. 23, 1987. Although the supernova itself is a million times fainter than 17 years ago, a new light show in the space surrounding it is just beginning.

This image, taken Nov. 28, 2003 by the Advanced Camera for Surveys aboard NASA's Hubble Space Telescope, shows many bright spots along a ring of gas, like pearls on a necklace. These cosmic "pearls" are being produced as a supersonic shock wave unleashed during the explosion slams into the ring at more than a million miles per hour. The collision is heating the gas ring, causing its innermost regions to glow. Curiously, one of the bright spots on the ring [at 4 o'clock] is a star that happens to lie along the telescope's line of sight.

Q & A: Understanding the Discovery1.Why did the star explode?

Stars at least 10 times more massive than our Sun end their lives in supernova explosions, among the most violent events in nature. When a massive star exhausts its nuclear fuel, its core collapses, sending a shock wave that tears apart the star.

2.Will our Sun end its life in a supernova blast?

No, our Sun is a medium-mass star that will end its life quietly by puffing away its outer layers of matter. As these layers of gas expand into space, they begin to glow, and are called a planetary nebula. A dense, glowing stellar remnant, called a white dwarf, is left behind.

3.Is anything left after a star explodes?

A neutron star or a black hole is formed in a supernova blast. Astronomers, however, have not detected either object in Supernova 1987A, either because it does not exist or the debris is so dense that astronomers cannot see inside the material.

4.How did the ring form?

One scenario is that the star cast off a disk of material into space as it devoured a stellar companion. The ring formed about 20,000 years before the doomed star exploded. A pair of bizarre outer rings also may have been cast off by the star before the explosion.

5.Why didn't the ring blow apart when the star exploded?

The ring was heated by a flash of ultraviolet light just after the stellar blast, causing the ring to glow. But the energetic shock wave unleashed by the exploding star is just arriving at the ring. The shock wave's powerful punch is creating a spectacular light show.